1. Field of the invention
This invention relates to a biaxially oriented polyester film for magnetic recording media, more specifically to a biaxially oriented polyester film for magnetic recording media, which contains alumina particles or colloidal silica particles having particular charactristics as well as heat resistant high molecular particles, wherein by protrusions formed on the film surface by these particles slipperiness, shaving resistance and scratch resistance are improved.
2. Description of the Prior Art
Biaxially oriented polyester films represented by polyethylene terephthalate films are widely used for magnetic recording media, namely as a base film for media, because of their excellent physical and chemical characteristics.
In biaxially oriented polyester films, their slipperiness and shaving resistance are important factors exerting an influence on whether workability is good or bad in the preparation steps or processing steps or on the quality of products therefrom. When these properties are insufficient, for example in case where a biaxially oriented polyester film is used as a magnetic tape by applying thereon a magnetic layer, friction between the coating roller and the film surface is large, and thus shaved powder is formed and scratches are formed on the film surface. Further, when such a polyester film is used as a base film for magnetic tapes for VTR or data cartridge, shaved powder and scrathes are formed in the step where these tapes are taken up in a high speed in cassettes, and such formation becomes a cause of omission of signal (D/0).
For the purpose of improvement of the slipperiness and shaving resistance of films, a method is adopted in general which comprises reducing the contact area between the film and the guide roller or the like by giving the film surface unevenness. Such methods are roughly classified into (i) a method which comprises depositing on the film surface inert particles from the catalyst residue of the polyester using as a film raw material, and (ii) a method which comprises adding inert inorganic particles. In general, the larger the size of the fine particles in these raw material polyesters is, the larger the improvement effect of slipperiness is. However, in magnetic tape, particularly in precision use such as video use, it is necessary that the unevenness of the film surface is as minute as possible because it could directly be a cause of occurrence of drawbacks such as drop out that the particles are large, and thus it is the state of things that request to satisfy opposite characteristics at the same time is made.
For improvement in the slipperiness of a film, the prior art discloses a method in which inorganic particles such as silicon oxide, titanium dioxide, calcium carbonate, talc, clay and calcined kaolin are added to a polyester used as a film substrate (see, for example, Japanese Laid-Open Patent Publication No. 57562/1979), and a method in which fine particles containing calcium, lithium or phosphorus are deposited within the polymerization system for producing a polyester (see, for example, Japanese Patent Publication No. 32914/17).
In film formation, the above fine particles insoluble in the polyester form protrusions on the film surface, and increase the slipperinces of the film. The method of improving the slipperiness of the film by providing protrusions of fine particles, however, gives cise to the essential problem that the protrusions, on the other hand, impair the flatness of the film surface. Attempts have been made to provide a compromise between flatness and slipperiness by utilizing a combination of fine particles having a relatively large particle size and fine particles having a relatively small particles size.
U. S. Pat. No. 3,821,156 discloses a combination of 0.02 to 0.1% by weight of calcium carbonate having an average particle size of 0.5 to 30 microns and 0.01 to 0.5% by weight of silica or hydrated aluminum silicate having an average particle size of 0.01 to 1.0 microns.
U. S. Pat. No. 3,884,870 discloses a combination of about 0.002 to about 0.018% by weight of fine inert particles, such as calcium carbonate, calcined aluminum silicate, hydrated aluminum silicate, magnesium silicate, calcium silicate, calcium phosphate, silica, alumina, barium sulfate, mica and diatomaccous earth, having a particle size of about 0.5 to about 30 microns and about 0.3 to about 2.5% by weight of fine inert particles, such as silica, calcium carbonate, calcined calcium silicate, hydrated calcium silicate, calcium phosphate, alumina, barium sulfate, magnesium, sulfate and diatomaceous earth, having a particle size of about 0.01 to about 1.0 micron.
U. S. Pat. No. 3,980,611 discloses that fine particles of calcium phosphate having three particle size grades of less than 1.0 micron, 1 to 2.5 microns and more than 2.5 microns are combined, and added to a polyester in a total amount of not more than 5,000 ppm.
Japanese Patent Publication No. 41,648/1980 (Laid-Open Patent Publication No. 71,154/1978) proposes a combination of 0.22 to 1.0% by weight of fine particles having a particle size of 1.2 to 2.5 microns and 0.003 to 0.25% by weight of fine particles having a particle size of 1.8 to 10 microns, the fine particles being those of an oxide or inorganic salt of an element of Groups II, III and IV of the periodic table.
Japanese Patent Publication No. 40,929/1980 (Laid-Open Patent Publication No. 11,908/1977) discloses a combination of 0.01 to 0.08% by weight of fine inert inorgenic particles having a particle size of 3 to 6 microns and 0.08 to 0.3% by weight of fine inert inorganic particles having a particle size of 1 to 2.5 microns, the total amount of these fine particles having different particles sizes being 0.1 to 0.4% by weight and the ratio of the large size particles to the small size particles being from 0.1 to 0.7.
Japanese Laid-Open Patent Publication No. 78,953/1977 discloses a biaxially oriented polyester film containing 0.01 to 0.5% by weight of inert particles having a particle size of 10 to 1,000 microns and 0.11 to 0.5% by weight of calcium carbonate having a particle size of 0.5 to 15 microns. This patent document lists up various inorganic substances other than calcium carbonate as the inert particles having a particle size of 10 to 1,000 microns in its general description, but only discloses specific examples in which silica or clay normally available as fine particles having a particle size of 10 to 1,000 microns is used as an inorganic substance.
U. S. Pat. No. 4,761,327 discloses a biaxially oriented polyester containing 0.005 to 1% by weight of silicone resin particles having an average particle size of 0.3 to 4 .mu.m and 0.005 to 1% by weight of inert particles having an average particle size of 0.01 to 1 .mu.m wherein the average particle size is smaller than the average particle size of the silicone resin particles.
Incidentally, such cases are lately increasing that, for the purpose of cost reduction, in guide posts fixed in cassettes, particularly in VTR cassettes metal guides whose surface is not sufficiently finished or plastic guides are used in place of the usual metal guides whose surface is sufficiently finished. However, the surface of these guide posts is extremely rough. thus when, in the case of magnetic tapes wherein a back coat is not provided, the usual technique to enhance the slipperiness and shaving property of the films is applied to such tapes, wherein shaved power and scratches formed in the step to take up the tapes in cassettes at a high speed and D/O level is heightened; therefore an improvement is strongly desired.